As disclosed in Japanese Laid-Open Patent Application No. 4-121260, there is known a braking force control apparatus which produces an increased braking force larger than a braking force during a normal control, when it is detected that an emergency braking operation is performed on an automotive vehicle. The conventional apparatus of the above publication is provided with a brake booster which produces a boosted pressure in response to a braking operation force Fp on a brake pedal of the vehicle, or the boosted pressure being equal to the braking operation force Fp multiplied by a given magnification factor. The boosted pressure is delivered from the brake booster to a master cylinder. The master cylinder produces a master cylinder pressure Pmc in response to the boosted pressure delivered from the brake booster, and the master cylinder pressure Pmc is proportional to the braking operation force Fp.
Further, the conventional apparatus of the above publication is provided with a high-pressure supplying unit having a pump which produces a brake-assist pressure. The high-pressure supplying unit produces a brake-assist pressure in accordance with a drive signal supplied by a control circuit. When a speed of the braking operation of the brake pedal exceeds a reference speed, it is determined that an emergency braking operation is performed by a vehicle operator, and the control circuit supplies a drive signal to the high-pressure supplying unit, the drive signal requesting a maximum brake-assist pressure to be produced by the high-pressure supplying unit. Both the brake-assist pressure produced by the high-pressure supplying unit and the master cylinder pressure Pmc produced by the master cylinder are supplied to a switching valve, and the switching valve delivers a larger one of the brake-assist pressure and the master cylinder Pmc to wheel cylinders of the vehicle.
In the conventional apparatus of the above publication, when the speed of the braking operation is below the reference speed, the master cylinder pressure Pmc, which is proportional to the braking operation force Fp, is supplied to the wheel cylinders. Hereinafter, the control that is performed to produce the braking force by the braking operation under such a condition will be called a normal control. On the other hand, when the speed of the braking operation is above the reference speed, the brake-assist pressure, which is produced by the high-pressure supplying unit, is supplied to the wheel cylinders. Hereinafter, the control that is performed to produce an increased braking force larger than the braking force produced during the normal control, under such a condition, will be called a brake-assist control.
In the conventional apparatus of the above publication, when the braking operation of the brake pedal is performed at a normal speed, the braking force is controlled to the magnitude that is proportional to the braking operation force Fp, and, when the emergency braking operation of the brake pedal is performed, the braking force is quickly increased to be larger than the braking force during the normal control.
In a certain running situation of the vehicle, the operator may perform a brake releasing operation of the brake pedal immediately after an emergency braking operation is performed. For example, when the vehicle comes into a situation that requires an emergency braking operation but the situation is shortly avoided, the operator tends to perform the brake releasing operation in such a situation.
In a case in which the brake releasing operation is performed by the operator immediately after the emergency braking operation is performed, the conventional apparatus of the above publication starts performing the brake-assist control at the time the emergency braking operation (the speed of which is above the reference speed) is performed. By the brake-assist control, the brake-assist pressure, larger than the brake fluid pressure produced during the normal control, is supplied from the high-pressure supplying unit to the wheel cylinders.
However, in the conventional apparatus of the above publication, during the brake-assist control, the increased brake fluid pressure is supplied from the high-pressure supplying unit to the wheel cylinders regardless of whether the stroke amount of the brake pedal is large or small. Hence, even after the brake releasing operation is performed by the operator, the conventional apparatus of the above publication continues to perform the brake-assist control. Although the situation that requires the emergency braking operation is avoided, the increased brake fluid pressure is continuously supplied to the wheel cylinders. An excessively large level of the braking force is continuously exerted on the vehicle in spite of the brake releasing operation, and this does not meet the operating feeling of the operator.